Polyoxymethylene resins have been widely used in automotive parts and electric and electronic parts because of their well-balanced mechanical properties and excellent fatigue characteristics. While the quality of polyoxymethylene resins has improved with increasing diversity of uses, conventional polyoxymethylene resins have unsatisfactory heat stability, giving rise to deterioration in appearance of moldings due to adhered matters to a mold (mold deposit). The poor heat stability of polyoxymethylene resins is ascribed to oxidative decomposition of the resin with a trace amount of oxygen to increase the amount of formaldehyde of decomposition at the time of molding. It is considered that the generated formaldehyde is converted to formic acid, which accelerates decomposition of the main chain of the polyoxymethylene resin.
As an approach to solving this problem, JP-B-43-14329 (the term "JP-B" as used herein means an "examined published Japanese patent application") and U.S. Pat. No. 3,219,727 propose to use an acrylamide polymer as a scavenger of formaldehyde generated by decomposition of a polyoxymethylene resin. However, the polyoxymethylene resin compositions disclosed in the two publications above are still unsatisfactory in heat stability and weatherability. Besides, neither of the publications has a mention of the importance of an acid imide group as used in the present invention. JP-B-6-10259 discloses a .beta.-alanine copolymer as a formaldehyde scavenger but has no mention of improvement on heat stability and weatherability of a polyoxymethylene resin by the existence of an acid imide group that is a characteristic point of the present invention. Further, JP-A-3-28260 (corresponding to U.S. Pat. No. 5,011,890 and EP-A-388809) discloses a composition comprising polyoxymethylene and polyacrylamide fine particles. Although the composition disclosed exhibits somewhat improved heat stability under nitrogen, the heat stability is still unsatisfactory in the presence of oxygen, i.e., under actual molding conditions, as will be verified in Comparative Examples 1 and 2 hereinafter given. This publication relates to a vinyl polymer having an amide group or a hydroxyl group, giving no suggestion with reference to the importance of an acid imide group characteristic of the present invention.
An object of the present invention is to provide a polyoxymethylene resin composition excellent in not only heat stability in the presence of oxygen but weatherability.